Toilet Smells Horrific When Flushing

Those exhaust fans plain ,and simple won't cause that sort of odor, that does sound like venting gone haywire. Sure if you stand there with the toilet flushing for too long you are going to pull some odor up, but normally that fan can run ,and flush fairly quick its going to be okay. Sure sometimes you will get a faint odor, but nothing like the OP is describing.